Extension of physical downlink control signaling in a communication system

ABSTRACT

Methods and apparatus are provided for the transmission of physical downlink control signaling. In one method, a User Equipment (UE) receives information associated with a physical resource block (PRB) for enhanced physical downlink control channel (EPDCCH) from a base station. Based on either first information received on physical control format indicator channel (PCFICH) or second information received on higher layer signaling, the UE determines a starting orthogonal frequency division multiple (OFDM) symbol on which EPDCCH transmission starts. The UE receives control information on the EPDDCH based on the information associated with the PRB and the determined starting OFDM symbol.

PRIORITY

The present application is a Continuation Application of, and claimspriority under 35 U.S.C. §120 to, U.S. patent application Ser. No.14/554,999, filed on Nov. 26, 2014 and issuing on Jul. 28, 2015 as U.S.Pat. No. 9,094,968, which was a Continuation Application of, and claimedpriority under 35 U.S.C. §120 to, U.S. patent application Ser. No.13/524,548, filed on Jun. 15, 2012 and issued on Dec. 30, 2014 as U.S.Pat. No. 8,923,201, which claimed priority under 35 U.S.C. §119(e) toU.S. Provisional Application Nos. 61/497,330 and 61/591,067, which werefiled in the United States Patent and Trademark Office on Jun. 15, 2011and Jan. 26, 2012, respectively. The contents of all of the above areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to wireless communicationsystems and, more particularly, to the transmission of physical downlinkcontrol signaling.

2. Description of the Art

A communication system includes a DownLink (DL) that conveystransmission signals from transmission points, such as Base Stations(BS) or NodeBs to User Equipments (UEs), and an UpLink (UL) that conveystransmission signals from UEs to reception points such as the NodeBs. AUE, also commonly referred to as a terminal or a mobile station, may befixed or mobile and may be a cellular phone, a personal computer device,etc. A NodeB is generally a fixed station and may also be referred to asan access point or some other equivalent terminology.

DL signals include data signals carrying information content, controlsignals, and Reference Signals (RSs), which are also known as pilotsignals. A NodeB conveys data signals to UEs through Physical DownlinkShared CHannels (PDSCHs) and control signals to UEs through PhysicalDownlink Control CHannels (PDCCHs). UL signals also include datasignals, control signals, and RS. UEs convey data signals to NodeBsthrough Physical Uplink Shared CHannels (PUSCHs) and control signals toNodeBs through Physical Uplink Control CHannels (PUCCHs). It is possiblefor a UE having transmission of data information to also convey controlinformation through the PUSCH.

Downlink Control Information (DCI) serves several purposes and isconveyed through DCI formats transmitted in PDCCHs. For example, DCIincludes DL Scheduling Assignments (SAs) for PDSCH reception and UL SAsfor PUSCH transmission. Because PDCCHs are a major part of a total DLoverhead, their resource requirements directly impact the DL throughput.One method for reducing PDCCH overhead is to scale its size according tothe resources required to transmit the DCI formats during a DLTransmission Time Interval (TTI). Assuming Orthogonal Frequency DivisionMultiple (OFDM) as the DL transmission method, a Control Channel FormatIndicator (CCFI) parameter transmitted through the Physical ControlFormat Indicator CHannel (PCFICH) can be used to indicate the number ofOFDM symbols occupied by the PDCCHs in a DL TTI.

FIG. 1 illustrates a conventional structure for PDCCH transmissions in aDL TTI.

Referring to FIG. 1, a DL TTI is assumed to consist of one subframehaving N=14 OFDM symbols. A DL control region including the PDCCHtransmissions occupies a first M OFDM symbols 110, i.e., M=3. Aremaining N-M OFDM symbols are used primarily for PDSCH transmissions120, i.e., M-N=9. A PCFICH 130 is transmitted in some sub-carriers, alsoreferred to as Resource Elements (REs), of a first OFDM symbol andincludes 2 bits indicating a DL control region size, e.g., M=1, M=2, orM=3 OFDM symbols.

For two NodeB transmitter antennas, some OFDM symbols also includerespective RS REs 140 and 150. These RSs are transmitted substantiallyover an entire DL operating BandWidth (BW) and are referred to as CommonRSs (CRSS) as they can be used by each UE to obtain a channel estimatefor its DL channel medium and to perform other measurements. Herein, aPDCCH transmitted with the conventional structure illustrated in FIG. 1will be referred to as a cPDCCH.

Additional control channels may be transmitted in a DL control region,but they are not shown for brevity. For example, assuming the use of aHybrid Automatic Repeat reQuest (HARM) process for data transmission ina PUSCH, a NodeB may transmit a Physical Hybrid-HARQ Indicator CHannel(PHICH) to indicate to UEs whether or not their previous PUSCHtransmissions were correctly received.

FIG. 2 illustrates a conventional encoding process for a DCI format.

Referring to FIG. 2, a NodeB separately codes and transmits each DCIformat in a respective PDCCH. A Radio Network Temporary Identifier(RNTI) for a UE for which a DCI format is intended masks the CyclicRedundancy Check (CRC) of a DCI format codeword in order to enable theUE to identify that the particular DCI format is intended for it. Forexample, both the CRC and the RNTI have 16 bits. The CRC 220 of the(non-coded) DCI format bits 210 is computed and it is subsequentlymasked 230 using the exclusive OR (XOR) operation between the CRC andRNTI bits 240. Accordingly, XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, andXOR(1,1)=0.

Thereafter, the masked CRC is appended to the DCI format informationbits 250, channel coding is performed 260, e.g., using a convolutionalcode, and rate matching 270 is performed to the allocated resources.Interleaving and modulation 280 is performed, and a control signal 290then transmitted.

FIG. 3 illustrates a conventional decoding process for a DCI format.

Referring to FIG. 3, a UE receiver performs the reverse operations of aNodeB transmitter to determine if the UE has a DCI format assignment ina DL subframe.

Specifically, a received control signal 310 is demodulated and theresulting bits are de-interleaved 320, a rate matching applied in aNodeB transmitter is restored 330, and data is subsequently decoded 340.After decoding, DCI format information bits 360 are obtained afterextracting CRC bits 350, which are then de-masked 370 by applying theXOR operation with a UE RNTI 380. Finally, a UE performs a CRC test 390.If the CRC test passes, a UE considers a DCI format to be valid anddetermines parameters for signal reception or signal transmission. Ifthe CRC test does not pass, a UE disregards the DCI format.

The DCI format information bits correspond to several fields, orInformation Elements (IEs), e.g., the Resource Allocation (RA) IEindicating the part of the operating BandWidth (BW) allocated to a UEfor PDSCH reception or PUSCH transmission, the Modulation and CodingScheme (MCS) IE indicating the data MCS, the IE related to the HARQoperation, etc. The BW unit for PDSCH or PUSCH transmissions is assumedto consist of several REs, e.g., 12 REs, and will be referred to hereinas a Resource Block (RB). Additionally, a RB over one subframe will bereferred to as a Physical RB (PRB).

To avoid a cPDCCH transmission to a UE blocking a cPDCCH transmission toanother UE, the location of each cPDCCH transmission in thetime-frequency domain of a DL control region is not unique and, as aconsequence, each UE performs multiple decoding operations to determinewhether there are cPDCCHs intended for it in a DL subframe. The REscarrying each cPDCCH are grouped into conventional Control ChannelElements (cCCEs) in the logical domain. For a given number of DCI formatbits in FIG. 2, the number of cCCEs for a respective cPDCCH depends on achannel coding rate (Quadrature Phase Shift Keying (QPSK) is assumed asthe modulation scheme). A NodeB may use a lower channel coding rate andmore cCCEs for cPDCCH transmission to UEs experiencing low DLSignal-to-Interference and Noise Ratio (SINR) than to UEs experiencing ahigh DL SINR. The cCCE aggregation levels include, for example, 1, 2, 4,and 8 cCCEs.

For a cPDCCH decoding process, a UE may determine a search space forcandidate cPDCCH transmissions after restoring the cCCEs in the logicaldomain according to a common set of cCCEs for all UEs (UE-Common SearchSpace or UE-CSS) and according to a UE-dedicated set of cCCEs(UE-Dedicated Search Space or UE-DSS). For example, the UE-CSS includesthe first C cCCEs in the logical domain. The UE-DSS may be determinedaccording to a pseudo-random function having as inputs UE-commonparameters, such as the subframe number or the total number of cCCEs inthe subframe, and UE-specific parameters such as the RNTI. For example,for cCCE aggregation levels L ε {1,2,4,8}, the cCCEs corresponding tocPDCCH candidate m are given by Equation (1).

cCCEs for cPDCCH candidate m=L·{(Y _(k) +m)mod└N _(CCE,k) /L┘}+i  (1)

In Equation (1), N_(CCE,k) is the total number of cCCEs in subframe k,i=0, . . . , L−1, m=0, . . . , M_(C) ^((L))−1, and M_(C) ^((L)) is thenumber of cPDCCH candidates to monitor in the search space. Exemplaryvalues of M_(C) ^((L)) for L ε {1,2,4,8} are {6, 6, 2, 2}, respectively.For the UE-CSS, Y_(k)=0. For the UE-DSS, Y_(k)=(A·Y_(k−1)) mod D, whereY⁻¹=RNTI≠0, A=39827, and D=65537.

DCI formats conveying information to multiple UEs are transmitted in aUE-CSS. Additionally, if enough cCCEs remain after the transmission ofDCI formats conveying information to multiple UEs, a UE-CSS may alsoconvey some DCI formats for DL SAs or UL SAs. A UE-DSS exclusivelyconveys DCI formats for DL SAs or UL SAs. For example, a UE-CSS mayinclude 16 cCCEs and support 2 DCI formats with L=8 cCCEs, 4 DCI formatswith L=4 cCCEs, 1 DCI format with=8 cCCEs, or 2 DCI formats with L=4cCCEs. The cCCEs for a UE-CSS are placed first in the logical domain(prior to interleaving).

FIG. 4 illustrates a conventional transmission process for cPDCCHs.

Referring to FIG. 4, after channel coding and rate matching, asillustrated in FIG. 2, the encoded DCI format bits are mapped, in thelogical domain, to cCCEs 400 of a cPDCCH. The first 4 cCCEs (L=4), i.e.,cCCE1 401, cCCE2 402, cCCE3 403, and cCCE4 404, are used for cPDCCHtransmission to UE1. The next 2 cCCEs (L=2), i.e., cCCE5 411 and cCCE6412, are used for cPDCCH transmission to UE2. The next 2 cCCEs (L=2),i.e., cCCE7 421 and cCCE8 422, are used for cPDCCH transmission to UE3.Finally, the last cCCE (L=1), i.e., cCCE9 431, is used for cPDCCHtransmission to UE4.

The DCI format bits are scrambled by a binary scrambling code in step440 and are subsequently modulated in step 450. Each cCCE is furtherdivided into mini-cCCEs or Resource Element Groups (REGs). For example,a cCCE including 36 REs can be divided into 9 REGs, each having 4 REs.Interleaving is applied among REGs (blocks of 4 QPSK symbols) in step460. For example, a block interleaver may be used where interleaving isperformed on symbol-quadruplets (4 QPSK symbols corresponding to the 4REs of a REG) instead of on individual bits.

After interleaving the REGs, a resulting series of QPSK symbols may beshifted by J symbols in step 470, and finally, each QPSK symbol ismapped to an RE in a DL control region in step 480. Therefore, inaddition to RSs 491 and 492 from NodeB transmitter antennas, and othercontrol channels such as a PCFICH 493 and a PHICH (not shown), REs in aDL control region include QPSK symbols for cPDCCHs corresponding to DCIformats for UE1 494, UE2 495, UE3 496, and UE4 497.

A UE may transmit an ACKnowledgement signal associated with a HARQprocess (HARQ-ACK signal) in a PUCCH in response to a reception of oneor more data Transport Blocks (TBs) in a PDSCH. When a PDSCH isscheduled by a DL SA in a respective cPDCCH, a UE may implicitly derivea PUCCH resource η_(PUCCH) for a HARQ-ACK signal transmission from theindex of a first cCCE, η_(CCE), of a respective cPDCCH transmission.Therefore, for a PDSCH reception in a given DL subframe, a UE determinesa PUCCH resource for an associated HARQ-ACK signal transmission in asubsequent UL subframe as η_(PUCCH)=f(η_(CCE)), where f( ) is a functionproviding a one-to-one mapping between a cCCE number and a PUCCHresource.

For example, f(η_(CCE))=η_(CCE)+N_(PUCCH), where N_(PUCCH) is an offseta NodeB informs to UEs by Radio Resource Control (RRC) signaling. If aUE is to determine multiple PUCCH resources for HARQ-ACK signaltransmission, resources associated with several consecutive cCCEs aftera first cCCE of a respective cPDCCH are used. For example, a secondPUCCH resource may be obtained from f(η_(CCE)+1). A UE can determine thetotal number of cCCEs used to transmit cPDCCHs in a subframe afterdecoding the PCFICH as, for a predetermined configuration of CRS REs,PHICH REs, and PCFICH REs, the number of cCCEs can be uniquelydetermined from the number of respective OFDM symbols.

The cPDCCH structure illustrated in FIG. 4 uses a maximum of M=3 OFDMsymbols and transmits a control signal over an operating DL BW.Consequently, THE cPDCCH structure has limited capacity and cannotachieve interference co-ordination in the frequency domain.

There are several cases in which expanded capacity or interferenceco-ordination in the frequency domain is used for PDCCH transmissions.One such case is a communication system with cell aggregation, where theDL SAs or UL SAs to UEs in multiple cells are transmitted in a singlecell (for example, because other cells may convey only PDSCH). Anothercase is extensive use of spatial multiplexing for PDSCH transmissionswhere multiple DL SAs correspond to same PDSCH resources. Another caseis when DL transmissions from a first NodeB experience stronginterference from DL transmissions from a second NodeB and DLinterference co-ordination in the frequency domain between the two cellsis needed.

A direct extension of a maximum DL control region size to more than M=3OFDM symbols is not possible at least due to the requirement to supportUEs which cannot be aware of such extension. Accordingly, a conventionalalternative is to extend a DL control region in a PDSCH region and useindividual PRBs for transmissions of control signals. Herein, a PDCCHtransmitted in this manner will be referred to as enhanced PDCCH(ePDCCH).

FIG. 5 illustrates a conventional use of PRBs for ePDCCH transmissionsin a DL TTI.

Referring to FIG. 5, although ePDCCH transmissions start immediatelyafter cPDCCH transmissions 510 and are over all remaining DL subframesymbols, alternatively, they may start at a fixed location, such as thefourth OFDM symbol, and extend over a part of remaining DL subframesymbols. The ePDCCH transmissions occurs in four PRBs, 520, 530, 540,and 550, while remaining PRBs may be used for PDSCH transmissions 560,562, 564, 566, and 568.

An ePDCCH reception may be based on a CRS or on a DemoDulation RS(DMRS). The DMRS is UE-specific and is transmitted in a subset of REs inPRBs used for an associated ePDCCH transmission.

FIG. 6 illustrates a conventional structure for DMRS REs in a PRBassociated with a PDSCH.

Referring to FIG. 6, DMRS REs 610 are placed in a PRB. For two NodeBtransmitter antenna ports, a DMRS transmission from a first antenna portis assumed to apply an Orthogonal Covering Code (OCC) of { 1, 1} overtwo DMRS REs located in a same frequency position and are successive inthe time domain, while a DMRS transmission from a second antenna port isassumed to apply an OCC of { 1, −1}. A UE receiver estimates a channelexperienced by a signal from each NodeB transmitter antenna port byremoving a respective OCC.

Several aspects for a combined cPDCCH and ePDCCH operation in FIG. 5still need to be defined in order to provide a functional design. Oneaspect is a process for a UE to detect cPDCCHs and ePDCCHs. To avoidincreasing a UE decoding complexity and a probability that a UEincorrectly assumes a cPDCCH or an ePDCCH as intended for it (i.e., afalse CRC check), it is desirable that a total number of respectivedecoding operations is substantially the same as when a UE does notmonitor any ePDCCH transmissions (for example, as illustrated in FIG.1).

Another aspect is that for ePDCCH reception based on a DMRS, a desiredreliability of channel estimate should be ensured especially for UEsexperiencing low DL SINR and requiring highly reliable ePDCCHreceptions. Unlike the case with a CRS, time-domain interpolation acrossdifferent DL subframes may not be possible with a DMRS and, as an ePDCCHtransmission is assumed to be either in one PRB or in two or morenon-adjacent PRBs, frequency-domain interpolation across different PRBsmay also be impossible.

Another aspect is a PUCCH resource determination for a HARQ-ACK signaltransmission in response to a reception of TBs conveyed in a PDSCHscheduled by a respective DL SA transmitted in an ePDCCH.

Therefore, there is a need for an ePDCCH decoding process at a UE in acommunication system supporting both cPDCCHs and ePDCCHs.

There is another need for a UE to determine a PUCCH resource forHARQ-ACK signal transmission in response to a reception of data TBsconveyed in a PDSCH scheduled by a respective DL SA transmitted in anePDCCH.

Further, there is another need to enhance the reliability of a channelestimate provided by the DMRS in a PRB conveying ePDCCH beyond the oneobtained in a PRB conveying PDSCH.

SUMMARY OF THE INVENTION

The present invention has been designed to solve at least theaforementioned limitations and problems in the prior art and provide atleast the advantages described below.

According to one aspect of the present invention, a method for wirelesscommunication in a user equipment (UE) is provided, including receivinginformation associated with a physical resource block (PRB) for enhancedphysical downlink control channel (EPDCCH); determining a startingorthogonal frequency division multiple (OFDM) symbol in which EPDCCHtransmission starts, based on either first information received onphysical control format indicator channel (PCFICH) or second informationreceived on higher layer signaling; and receiving control information onthe EPDDCH based on the information associated with the PRB and thedetermined starting OFDM symbol.

According to another aspect of the present invention, a user equipment(UE) for wireless communication is provided, the UE including: areceiver configured to receive information associated with a physicalresource block (PRB) for enhanced physical downlink control channel(EPDCCH); and a search configured to determine a starting orthogonalfrequency division multiple (OFDM) symbol in which enhanced physicaldownlink control channel (EPDCCH) transmission starts, based on eitherfirst information received on physical control format indicator channel(PCFICH) or second information received on higher layer signaling,wherein the receiver is further configured to receive controlinformation on the EPDDCH based on the information associated with thePRB and the determined starting OFDM symbol.

According to yet another aspect of the present invention, a method forwireless communication in a base station is provided, including:transmitting information associated with a physical resource block (PRB)for enhanced physical downlink control channel (EPDCCH) and at least oneof first information on physical control format indicator channel(PCFICH) and second information on higher layer signaling; determining astarting orthogonal frequency division multiple (OFDM) symbol in whichEPDCCH transmission starts, based on either the first information on thePCFICH or the second information on the higher layer signaling; andtransmitting control information on the EPDDCH based on the informationassociated with the PRB and the determined starting OFDM symbol.

According to still another aspect of the present invention, a basestation for wireless communication is provided, including: a transmitterconfigured to transmit information associated with a physical resourceblock (PRB) for enhanced physical downlink control channel (EPDCCH) andat least one of first information on physical control format indicatorchannel (PCFICH) and second information on higher layer signaling; and aselector configured to determine a starting orthogonal frequencydivision multiple (OFDM) symbol in which EPDCCH transmission starts,based on either the first information on the PCFICH or the secondinformation on the higher layer signaling, wherein the transmitter isfurther configured to transmit control information on the EPDDCH basedon the information associated with the PRB and the determined startingOFDM symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a conventional structure for cPDCCHtransmissions;

FIG. 2 is a block diagram illustrating a conventional encoding processfor a DCI format;

FIG. 3 is a block diagram illustrating a conventional decoding processfor a DCI format;

FIG. 4 is a diagram illustrating a conventional transmission process forcPDCCHs;

FIG. 5 is a diagram illustrating a conventional use of PRBs for ePDCCHtransmissions;

FIG. 6 is a diagram illustrating a conventional structure for DMRS REsin a PRB associated with a PDSCH;

FIG. 7 is a flowchart illustrating a UE operation for cPDCCH detectionor for ePDCCH detection in response to an RRC configuration, accordingto an embodiment of the present invention;

FIG. 8 is a flowchart illustrating a UE operation for decoding cPDCCHcandidates and ePDCCH candidates, according to an embodiment of thepresent invention;

FIG. 9 is a diagram illustrating additional DMRS density structurescorresponding to an antenna port in PRBs conveying ePDCCHs compared toPRBs conveying PDSCHs, according to an embodiment of the presentinvention;

FIG. 10 is a diagram illustrating an ordering of cCCEs and of eCCEs forPUCCH resource determination for HARQ-ACK signal transmission, accordingto an embodiment of the present invention; and

FIG. 11 is a diagram illustrating an ordering of cCCEs and of eCCEs forPUCCH resource determination for HARQ-ACK signal transmission, accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Various embodiments of the present invention will now be described indetail hereinafter with reference to the accompanying drawings. Thepresent invention may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete and will fully convey the scope of the presentinvention to those skilled in the art.

Additionally, although the embodiments of the present invention will bedescribed below with reference to Orthogonal Frequency DivisionMultiplexing (OFDM), they also are applicable to all Frequency DivisionMultiplexing (FDM) transmissions in general and to Discrete FourierTransform (DFT)-spread OFDM in particular.

The embodiments of the invention do not assume a particular structurefor ePDCCH transmissions. The respective PRBs are generally assumed toinclude at least one CCE (eCCE), which may have a same size (number ofREs) as cCCEs for cPDCCH transmissions.

In a DL subframe, an eCCE size depends on a number of eCCEs per PRB, anexistence of various RS types, such as CRS or DMRS, in a PRB (therespective REs cannot be used for ePDCCH transmission), a number of OFDMsymbols used for ePDCCH transmissions, etc.

A PRB includes at least one ePDCCH transmission and an ePDCCHtransmission may be entirely included in one PRB or be distributed overmultiple PRBs.

An ePDCCH transmission may begin in an OFDM symbol immediately after thelast OFDM symbol of a conventional DL control region (determined by a UEafter decoding the PCFICH), or an ePDCCH transmission may begin at afixed OFDM symbol informed to a UE by higher layer signaling. Forexample, an ePDCCH transmission may begin at the OFDM symbol after theone corresponding to a maximum number of OFDM symbols used for theconventional DL control region. The number of OFDM symbols used forePDCCH transmissions may be all of the remaining OFDM symbols in a DLsubframe or any subset of these remaining OFDM symbols.

In accordance with an embodiment of the present invention, a UEdetection process is provided for cPDCCHs and ePDCCHs in a communicationsystem supporting their coexistence in a same DL subframe.

Specifically, a UE is informed through higher layer signaling, e.g., RRCsignaling, whether to decode only cPDCCH or only ePDCCH. For example,one bit of RRC signaling may be used for this purpose, i.e., a binary‘0’ indicates cPDCCH detection and a binary ‘1’ indicates ePDCCHdetection).

FIG. 7 illustrates a UE operation for cPDCCH detection or for ePDCCHdetection in response to an RRC configuration, according to anembodiment of the present invention.

Referring to FIG. 7, a NodeB signals to a UE whether to decode onlycPDCCH or only ePDCCH, using RRC signaling of 1 bit in step 710. A UEreceives RRC signaling from a NodeB in step 720, determines whether theRRC signaling indicates decoding only cPDCCH or only ePDCCH in step 730,and decodes only cPDCCH in step 740 or only ePDCCH in step 750, based onthe detection in step 730.

The above-described approach provides simplicity at the cost ofincreasing the blocking probability of cPDCCH or ePDCCH transmissionsand of increasing the probability of resource waste (smaller utilizationof respective available resources). For example, if a UE decodes onlyePDCCHs, the corresponding resources in allocated PRBs may be exhausteddue to ePDCCH transmissions to other UEs in a DL subframe. Therefore, anePDCCH transmission to a referenced UE is blocked and the UE is notscheduled in a DL subframe, even though there are available resourcesfor a NodeB to transmit cPDCCH to a referenced UE.

Further, if the allocation of PRBs for ePDCCH transmissions to UEs isconfigured by RRC signaling and each PRB includes several eCCEs used forePDCCH transmissions to a same UE or to different UEs, it is possiblethat only some of the eCCEs in a PRB are used and the remaining ones arewasted. In such a case, partial use of PRBs for ePDCCH transmissionscould be avoided if a UE was capable of detecting cPDCCH. Conversely, asthe granularity of a DL control region for cPDCCH transmissions isassumed to be one OFDM symbol, a whole OFDM symbol may be used only totransmit a few cCCEs to accommodate, for example, one additional cPDCCHtransmission to a UE. This additional cPDCCH transmission and a use ofan additional OFDM symbol could be avoided if a referenced UE was alsocapable of detecting ePDCCHs.

To address the above-described shortcomings of the embodiment above, inaccordance with another embodiment of the present invention a UE isprovided that can decode both cPDCCHs and ePDCCHs. The search spacestructure for ePDCCH decoding may not necessarily be exactly the same asthe one described, for example, for the UE-DSS for cPDCCH decoding, inEquation (1). However, a structure defining ePDCCH candidates M_(E)^((L)) for eCCE aggregation level L is again assumed. For simplicity,same aggregation levels L ε {1,2,4,8} for cPDCCH and ePDCCH decoding maybe assumed, but not required, as will be described below.

A number of decoding operations for cPDCCHs or ePDCCHs is determined bya respective number of candidates for each possible cCCE or eCEEaggregation level, respectively. This number can be either predeterminedor configured to a UE by a NodeB through RRC signaling. For example, forL ε {1,2,4,8}, a NodeB can configure a UE to perform an equal number ofdecoding operations for cPDCCH and ePDCCH and a total number of decodingoperations equal to the case a UE decodes, e.g., only cPDCCH, by settinga respective number of cPDCCH candidates as M_(E) ^((L)) ε {3,3,1,1} andsetting a respective number of ePDCCH candidates as M_(E) ^((L)) ε{3,3,1,1}. The cPDCCH candidates may be allocated to at least one of aUE-CSS or a UE-DSS.

Alternatively, a NodeB may prioritize either cPDCCH or ePDCCH decodingby a UE. For example, for L ε {1,2,4,8}, a NodeB can configure a UE withM_(C) ^((L)) ε {1,1,0,0} cPDCCH candidates and with M_(E) ^((L)) ε{5,5,2,2} ePDCCH candidates.

FIG. 8 illustrates a UE operation for decoding cPDCCH candidates andePDCCH candidates, according to an embodiment of the present invention.

Referring to FIG. 8, a number of cPDCCH candidates and a number ofePDCCH candidates for each possible cCCE or eCCE aggregation level,respectively, is either configured to a UE by a NodeB through RRCsignaling or is predetermined. In the former case, a NodeB signals to aUE a number of cPDCCH candidates M_(C) ^((L)) and a number of ePDCCHcandidates M_(E) ^((L)) for each cCCE and eCCE aggregation level L instep 810. A UE receives signaling from a NodeB in step 820, determines,using for example Equation (1), each possible cPDCCH candidate andePDCCH candidate for a respective cCCE and eCCE aggregation level L instep 830, and performs associated decoding operations in step 840.

In accordance with another embodiment of the invention, ePDCCH detectionreliability is enhanced by basing demodulation on a DMRS, instead of aCRS.

The DMRS design in FIG. 6 is targeted for PDSCH demodulation for which atarget error rate is much larger than a target error rate of ePDCCH,typically by at least an order of magnitude. Further, a PDSCH can relyon HARQ retransmissions for an eventual correct reception of a TB. Dueto the more stringent requirements for ePDCCH reception reliability andin order to avoid increasing a code rate by using more eCCEs for anePDCCH transmission, thereby increasing a respective overhead, it mayoften be preferable to provide a UE the ability to improve a reliabilityfor an estimate of a channel experienced by an ePDCCH, thereby improvingePDCCH detection reliability. Further, for a largest eCCE aggregationlevel L, e.g., L=8 eCCEs, it may not be possible to increase the numberof eCCEs allocated to an ePDCCH. It is for UEs experiencing very low DLSINRs that a largest eCCE aggregation level is used but also thatchannel estimation accuracy is most important.

For the DMRS design illustrated in FIG. 6, PRBs conveying ePDCCH have alarger density of DMRS (more DMRS REs) for a respective antenna port inthe frequency domain, the time domain, or in both domains. Theadditional REs may be used to transmit additional DMRS from a respectiveantenna port or may remain empty and their power may be used to boostthe transmission power of existing DMRS from a respective antenna port.

FIG. 9 is a diagram illustrating additional DMRS density structurescorresponding to an antenna port in PRBs conveying ePDCCHs, compared toPRBs conveying PDSCHs, according to an embodiment of the presentinvention. Specifically, FIG. 9 illustrates additional DMRS densitystructures corresponding to an antenna port in PRBs conveying ePDCCHs,compared to PRBs conveying PDSCHs for which a DMRS density is assumed tobe as illustrated in FIG. 6.

Referring to FIG. 9, an increased DMRS density for a respective antennaport can be either in the time domain 910, the frequency domain 920, orin both the time domain and the frequency domain (for example, bycombining 910 and 920). A UE may then apply conventional methods, suchas time or frequency interpolation, to combine the additional DMRS REswith existing DMRS REs located in same positions as ones used for PDSCHdemodulation, or the additional DMRS REs may remain empty and theirrespective power may be used to boost the transmission power of DMRS inexisting REs.

Another alternative for improving an ePDCCH detection reliability is tohave a larger maximum eCCE aggregation level for ePDCCH than the maximumcCCE aggregation level for cPDCCH. For example, possible cCCEaggregation levels can be L ε {1,2,4,8} while possible eCCE aggregationlevels can be L ε {1,2,4,8,16}. Accordingly, the degradation in anePDCCH reception reliability, from using a DMRS-based demodulationversus using a CRS-based demodulation as for the cPDCCH, can becompensated by an effective doubling of a received ePDCCH power fromusing L=16 instead of L=8.

In accordance with another embodiment of the present invention, a PUCCHresource determination for HARQ-ACK signal transmission from a UE inresponse to a reception of TBs conveyed in a PDSCH scheduled by arespective DL SA transmitted in an ePDCCH is utilized. Transmissions ofHARQ-ACK signals associated with respective PDSCH receptions in a sameDL subframe are in a same UL subframe, regardless of whether a PDSCHreception was scheduled by a cPDCCH or an ePDCCH.

The same implicit rule for a PUCCH resource determination is assumed toapply, as when only cPDCCH is transmitted. As cPDCCH transmissionsalways occur in a DL subframe, while ePDCCH transmissions may or may notoccur, cCCEs may be ordered first with respect to a determination ofPUCCH resources for HARQ-ACK signal transmission. Moreover, a UE may notbe aware of an existence of an ePDCCH, if it is not configured forePDCCH reception by a NodeB.

In a first approach, PUCCH resources for HARQ-ACK signal transmissionscorresponding to ePDCCHs are consecutive to the ones corresponding tocPDCCHs. A UE determines a placement of PUCCH resources corresponding toePDCCHs either by decoding a PCFICH to determine a number of OFDMsymbols used for the transmission of cPDCCHs in a DL subframe or byconsidering a number of OFDM symbols for the transmission of cPDCCHs asinformed by higher layer signaling. In either case, the number of OFDMsymbols used for transmission of cPDCCHs in a DL subframe determines themaximum number of respective cCCEs.

FIG. 10 illustrates an ordering of cCCEs and of eCCEs for PUCCH resourcedetermination for HARQ-ACK signal transmission, according to anembodiment of the present invention.

Referring to FIG. 10, four PRBs are configured for potentialtransmissions of ePDCCHs 1010, 1012, 1014, and 1016. Each PRB includesfour eCCEs which, for example, are first numbered in the frequencydomain in ascending PRB order and then in the time domain (the eCCEs mayalternatively be first mapped in the time domain in ascending order ofPRBs). Assuming that a number of cCCEs corresponding to OFDM symbolsused for transmissions of cPDCCHs is N_(C), cCCEs are ordered first andPUCCH resources for respective HARQ-ACK signal transmissions aredetermined using the previously described conventional mapping withPUCCH resource η_(PUCCH)=f(η_(CCE,C)) 1020 corresponding to cCCE numberη_(CCE,C) 1030. Subsequently, eCCEs are mapped to PUCCH resources usedfor HARQ-ACK signal transmission with PUCCH resourceη_(PUCCH)=f(N_(C)+η_(CCE,E)) 1040 corresponding to eCCE numberη_(CCE,E), which is the first eCCE of a respective ePDCCH 1050.

Although FIG. 10 considers that each PRB allocated to transmissions ofePDCCHs extends over all OFDM symbols of the DL subframe, alternatively,a subset of these OFDM symbols from the beginning of the DL subframe,and after the OFDM symbols used for the transmission of cPDCCHs, may beused for transmissions of ePDCCHs.

In accordance with another embodiment of the present invention, insteadof PUCCH resources for HARQ-ACK signal transmissions corresponding toePDCCHs being consecutive to the ones corresponding to cPDCCHs, UEsconfigured to receive only ePDCCH may independently determine thesePUCCH resources by applying an offset to the PUCCH resourcescorresponding to cPDCCHs, e.g., by assuming a maximum PUCCH resourcescorresponding to cPDCCHs. This is advantageous, as a UE configured toreceive only ePDCCH does not decode a PCFICH (a UE configured for ePDCCHdecoding may also be configured by a NodeB through 1-bit RRC signalingwhether or not to decode a PCFICH). This is applicable when a UEexperiences poor DL SINK over an entire DL BW, due to inter-cellinterference, and is allocated ePDCCH in interference protected PRBs (aPCFICH is transmitted substantially over the entire DL

BW and cannot be protected from interference). The disadvantage is thatsome PUCCH resources will remain unused when a number of OFDM symbolsfor cPDCCHs is not a maximum one.

FIG. 11 illustrates an ordering of cCCEs and of eCCEs for PUCCH resourcedetermination for HARQ-ACK signal transmission, according to anembodiment of the present invention.

Referring to FIG. 11, four PRBs are configured for potentialtransmissions of ePDCCHs 1110, 1112, 1114, and 1116. Each PRB includesfour eCCEs which, for example, are first numbered in the frequencydomain in ascending PRB order and then in the time domain (the eCCEs mayalternatively be first mapped in the time domain in ascending order ofPRBs). A UE configured by a NodeB to receive only ePDCCH does not decodea PCFICH and assumes a fixed number of cCCEs, such as a maximum numberof cCCEs N_(C,max), by assuming that a maximum number of OFDM symbols isused for transmissions of cPDCCHs. Therefore, a UE configured by a NodeBto receive only ePDCCH assumes that N_(C,max) cCCEs are ordered firstand PUCCH resources for respective HARQ-ACK signal transmissions aredetermined using the previously described conventional mapping withPUCCH resource η_(PUCCH)=f(η_(CCE,C)) 1120 corresponding to cCCE numberη_(CCE,C) 1130. Subsequently, eCCEs are mapped to

PUCCH resources used for HARQ-ACK signal transmissions with PUCCHresource η_(PUCCH)=f(N_(C,max)+η_(CCE,E)) 1140 corresponding to eCCEnumber η_(CCE,E) 1150.

When a number of OFDM symbols used in a DL control region fortransmissions of cPDCCHs is less than a maximum one, PRBs in OFDMsymbols after the last one of a DL control region for transmissions ofcPDCCHs and up to a maximum possible one of a DL control region fortransmissions of cPDCCH 1160 are used for PDSCH transmissions in samePRBs to UEs configured to receive cPDCCH 1170, but are not used foreither ePDCCH or PDSCH transmissions in same PRBs to UEs configured toreceive ePDCCH 1180.

For example, if a DL control region of a DL subframe for transmission ofcPDCCHs uses M=1 OFDM symbol and a maximum possible number is three OFDMsymbols, the second and third OFDM symbols are used for transmission ofPDSCH to UEs configured to receive cPDCCH, but are not used fortransmission of PDSCH to UEs configured to receive ePDCCH.

Although FIG. 11 considers that each PRB allocated to transmissions ofePDCCHs extends over all OFDM symbols of a DL subframe, alternatively, asubset of these OFDM symbols from the beginning of a DL subframe, andafter the OFDM symbols used for the transmission of cPDCCHs, may be usedfor transmissions of ePDCCHs.

In accordance with another embodiment of the present invention, HARQ-ACKsignal transmissions from UEs in response to respective ePDCCHdetections share a same set of PUCCH resources with HARQ-ACK signaltransmissions from UEs in response to respective cPDCCH detections.Collisions are avoided by determining a PUCCH resource for a formerHARQ-ACK signal transmission as η_(PUCCH)f(η_(CCE,E),HRI), where HRI isa HARQ-ACK Resource Indicator (HRI) field included in DCI formatsconveyed by ePDCCHs scheduling PDSCHs (the HRI is not included in DCIformats conveyed by cPDCCHs scheduling PDSCHs).

For example, an HRI includes 2 bits where ‘00’ maps to −2, ‘01’ maps to−1, ‘10’ maps to 0, and ‘11’ maps to 1, andη_(PUCCH)=f(η_(CCE,E),HRI)=η_(CCE-E)+HRI+N_(PUCCH). This approach isdescribed in U.S. application Ser. No. 12/986,675, entitled “ResourceIndexing for Acknowledgement Signals in Response to Receptions ofMultiple Assignments,” which is incorporated by reference.

While the present invention has been shown and described with referenceto certain embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for receiving control information in auser equipment (UE), the method comprising: receiving higher layersignaling associated with a physical resource block (PRB) for enhancedphysical downlink control channel (EPDCCH); receiving the controlinformation on the EPDCCH of which transmission starts in a firststarting orthogonal frequency division multiplexing (OFDM) symboldetermined by the higher layer signaling, if the first starting OFDMsymbol is configured by the higher layer signaling; and receiving thecontrol information on the EPDCCH of which transmission starts in asecond starting OFDM symbol determined by a physical control formatindicator channel (PCFICH), if the first starting OFDM symbol is notconfigured by the higher layer signaling.
 2. The method of claim 1,wherein the control information is received by using a number ofenhanced control channel elements (ECCEs) in a downlink (DL) subframe.3. The method of claim 2, wherein a search space in which the number ofECCEs are monitored is determined according to a UE-dedicated searchspace (UE-DSS).
 4. The method of claim 1, further comprising: receivinga physical downlink shared channel (PDSCH) using a number of OFDMsymbols.
 5. The method of claim 4, wherein the number of OFDM symbols isconfigured by the higher layer signaling.
 6. A user equipment (UE) forreceiving control information, the UE comprising: a receiver configuredto receive higher layer signaling associated with a physical resourceblock (PRB) for enhanced physical downlink control channel (EPDCCH), toreceive the control information on the EPDCCH of which transmissionstarts in a first starting orthogonal frequency division multiplexing(OFDM) symbol determined by the higher layer signaling, if the firststarting OFDM symbol is configured by the higher layer signaling, and toreceive the control information on the EPDCCH of which transmissionstarts in a second starting OFDM symbol determined by a physical controlformat indicator channel (PCFICH), if the first starting OFDM symbol isnot configured by the higher layer signaling.
 7. The UE of claim 6,wherein the control information is received by using a number ofenhanced control channel elements (ECCEs) in a downlink (DL) subframe.8. The UE of claim 7, wherein a search space in which the number ofECCEs are monitored is determined according to a UE-dedicated searchspace (UE-DSS).
 9. The UE of claim 6, wherein the receiver is furtherconfigured to: receive a physical downlink shared channel (PDSCH) usinga number of OFDM symbols.
 10. The UE of claim 9, wherein the number ofOFDM symbols is configured by the higher layer signaling.
 11. A methodfor transmitting control information in a base station, the methodcomprising: transmitting, to a user equipment (UE), higher layersignaling associated with a physical resource block (PRB) for enhancedphysical downlink control channel (EPDCCH); transmitting the controlinformation on the EPDCCH of which transmission starts in a firststarting orthogonal frequency division multiplexing (OFDM) symboldetermined by the higher layer signaling, if the first starting OFDMsymbol is configured by the higher layer signaling; and transmitting thecontrol information on the EPDCCH of which transmission starts in asecond starting OFDM symbol determined by a physical control formatindicator channel (PCFICH), if the first starting OFDM symbol is notconfigured by the higher layer signaling.
 12. The method of claim 11,wherein the control information is transmitted by using a number ofenhanced control channel elements (ECCEs) in a downlink (DL) subframe.13. The method of claim 12, wherein a search space in which the numberof ECCEs are monitored is determined according to a UE-dedicated searchspace (UE-DSS).
 14. The method of claim 11, further comprising:transmitting a physical downlink shared channel (PDSCH) using a numberof OFDM symbols.
 15. The method of claim 14, wherein the number of OFDMsymbols is configured by the higher layer signaling.
 16. A base stationfor transmitting control information, the base station comprising: atransmitter configured to transmit, to a user equipment (UE), higherlayer signaling associated with a physical resource block (PRB) forenhanced physical downlink control channel (EPDCCH), to transmit thecontrol information on the EPDCCH of which transmission starts in afirst starting orthogonal frequency division multiplexing (OFDM) symboldetermined by the higher layer signaling, if the first starting OFDMsymbol is configured by the higher layer signaling, and to transmit thecontrol information on the EPDCCH of which transmission starts in asecond starting OFDM symbol determined by a physical control formatindicator channel (PCFICH), if the first starting OFDM symbol is notconfigured by the higher layer signaling.
 17. The base station of claim16, the control information is received by using a number of enhancedcontrol channel elements (ECCEs) in a downlink (DL) subframe.
 18. Thebase station of claim 17, wherein a search space in which the number ofECCEs are monitored is determined according to a UE-dedicated searchspace (UE-DSS).
 19. The base station of claim 16, wherein thetransmitter is further configured to: transmit a physical downlinkshared channel (PDSCH) using a number of OFDM symbols.
 20. The basestation of claim 19, wherein the number of OFDM symbols is configured bythe higher layer signaling.